A number of different data transmission protocols are in commercial use. While switching systems dedicated to particular protocols are common, it has been more difficult to produce general switching systems that are able to switch multiple protocols. This is partially due to the diverse formats used by the various protocols.
Digital loop carrier (DLC) systems utilize fiber-optic distribution links and remote multiplexing devices to deliver voice and data signals to and from the local users. In a typical DLC system, a fiber optic cable is routed from the central office terminal (COT) to a host digital terminal (HDT) located within a particular neighborhood. Telephone lines from subscriber homes are then routed to circuitry within the HDT, where the telephone voice signals are converted into digital pulse-code modulated (PCM) signals, multiplexed together using a time-slot interchanger (TSI), converted into an equivalent optical signal, and then routed over the fiber optic cable to the central office. Likewise, telephony signals from the central office are multiplexed together, converted into an optical signal for transport over the fiber to the HDT, converted into corresponding electrical signals at the HDT, demultiplexed, and routed to the appropriate subscriber telephone line twisted-pair connection.
In some communications systems or networks, such as a network core transport system supporting a digital loop carrier (DLC) system or other communications network, a device may transmit or receive communications signals with an associated clock in a source-synchronous manner.